The present invention relates to an electric rotating machine such as a stepping motor used in OA (Office Automation) equipment or the like. Specifically, the present invention relates to a hybrid type double three-phase electric rotating machine.
An electric rotating machine such as a stepping motor that has a high positioning accuracy and low vibration is required in a use of OA equipment or the like. Japanese patent gazette No. 3537904 (JP 3537904B) discloses a double three-phase stepping motor that has two sets of three-phase coils. The motor is high in resolution, suitable for a use of positioning and low in cost.
FIG. 9 is a section view in a direction perpendicular to a rotation axis of the stepping motor shown in the patent gazette. The motor 2 disclosed in the patent gazette is provided with a stator 3 that has twelve stator poles 3b extended inwardly and radially from a cylindrical-shaped yoke portion 3a and a rotor 4 arranged inside the tip ends of the stator poles 3b with a predetermined air gap therebetween.
Five inductors (pole teeth) are formed on the tip end of each the stator pole 3b of the stator 3. Further, the angle formed between center lines of adjacent two stator poles is equal to (30−γ) degrees or (30+γ) degrees. Angle of (30−γ) degrees and angle of (30+γ) degrees are alternately repeated. An excitation coil is wound around each the stator pole. A coil A1 is wound around the stator pole at the center top in the drawing, and coils a1, B1, b1, C1, c1, A2, a2, B2, b2, C2 and c2 are wound around the stator poles in the order of clockwise from the next of the coil A1.
A pair of coils wound around the stator poles being substantially symmetric with respect to a point form one coil set for one phase. Three coil sets A1 and A2, B1 and B2, C1 and C2 form one group for three phases, and three coil sets a1 and a2, b1 and b2, c1 and c2 form the other group for three phases.
As shown in FIG. 10, the coils of the same phase are connected in series between terminals so that a pair of stator poles around which the coils of the same phase are wound are excited in the same polarity when a voltage is applied between terminals.
The rotor 4 is a popular hybrid type in which a pair of rotor cores sandwich a permanent magnet. The rotor 4 is fixed to a rotation shaft 5 that is supported by bearings (not shown). Rotor teeth are formed around the rotor cores. The pitch of the rotor teeth is corresponding to that of the inductors of the stator pole. The permanent magnet magnetizes the rotor teeth of one rotor core in N-pole and the rotor teeth of the other rotor core in S-pole. The rotor cores are arranged so that the rotor teeth thereof are deviated in angular position by ½ pitch of the rotor teeth.
FIG. 11 is a conceptual illustration that shows a relationship between the inductors of the stator and the rotor teeth. FIG. 11 shows the center inductors of the respective stator poles of the stator 3 and the rotor teeth that are closest to the respective inductors. Here, the deviations between the inductors of the stator poles and the rotor teeth are shown in electrical angle. When the center inductors of the stator poles around which the coils A1 and A2 are wound just face to the rotor teeth (N-pole) of the first rotor core without angular deviation, the center inductor of the next stator pole around which the coil al is wound is deviated by 30 degrees in electric angle with respect to the rotor tooth (N-pole) of the first rotor core, the center inductor of the next stator pole around which the coil B1 is wound is deviated by 60 degrees in electric angle with respect to the rotor tooth (S-pole) of the second rotor core, and the center inductor of the next stator pole around which the coil b1 is wound is deviated by 90 degrees in electric angle with respect to the rotor tooth (S-pole) of the second rotor core. Assuming that the number of rotor teeth of each rotor core Nr equals 100, the stator poles are arranged so that the angle between adjacent poles becomes 29.1 degrees or 30.9 degrees that are alternately repeated.
Though the double three-phase stepping motor disclosed in the patent gazette has high resolution, cogging torque generated by the permanent magnet of the rotor when the coils are not excited becomes large, which causes large vibration and large noise under operation.